This invention relates to a method for preparing polyolefins having a multimodal molecular weight distribution.
Polyolefins having a multimodal molecular weight distribution (MWD), such as polyethylene, can be made into articles by a variety of methods, including, but not limited to, extrusion molding, thermoforming and rotational molding, and have advantages over typical polyolefins lacking a multimodal MWD. Polyolefins having a multimodal MWD process more easily, i.e., they can be processed at a faster throughput rate with lower energy requirements, and at the same time such polymers exhibit reduced melt flow perturbations and are preferred because of improved properties for applications such as blow molding and/or high strength films. Polymers having a multimodal MWD are generally characterized by having a broad MWD, or more that one MWD peak, as reflected by size exclusion chromatography (SEC) curves.
There are several known methods of producing polyolefins having a multimodal MWD; however, each method has its own disadvantages. Polyolefins having a multimodal MWD can be made by employing two distinct and separate catalyst systems in the same reactor, each producing a polyolefin having a different MWD; however, catalyst feed rates are usually difficult to control and the catalysts can have a detrimental effect on each other. Polymer particles produced from a dual, or even multi-, catalyst system frequently are not uniform in size. Thus, segregation of the polymer during storage and transfer can produce non-homogeneous products.
A polyolefin having a bimodal MWD can also be made by sequential polymerization in two or more separate reactors or blending polymers of different MWD during processing; however, both of these methods increase capital cost, and problems discussed earlier regarding polymer segregation can occur.
Multimodal MWD polyethylenes can also be obtained directly from a single reactor polymerization process in the presence of a catalyst system comprising two or more catalytic sites, such as, for example, metallocenes, wherein each site has different propagation and termination rate constants. At certain ratios, and in certain polymerization processes, even catalysts that have two different catalytic sites can produce a monomodal, or narrow, MWD polyolefin. Unfortunately, even under ideal conditions, a dual site catalyst system can have decreased catalytic activity. While not wishing to be bound by theory, it is hypothesized that a metallocene can bind to, and therefor inhibit the reactivity of, some of the active chromium oxide catalytic sites. Unfortunately, there are limits to known methods of preparing these very desirable, bimodal molecular weight distribution or multimodal molecular weight distribution polyolefins.